Overhead power lines comprise overhead power conductors spliced together with overhead connectors. Electrical power passes from a conductor into the connector and into the next conductor. The overhead connectors are a weak link in the system. Due to failure of these overhead connectors, the reliability of overhead power lines has become an impediment to increasing the electrical power flow through the electrical grid.
Overhead power connectors may fail as a result of improper installation, corrosion, or age. These problems lead to increased resistance through the connector and, therefore, an increase in temperature. The connectors may overheat eventually resulting in catastrophic failure of the connector, loss of transmission of electricity through the conductor, and significant electrical and impact hazards to line crews and anyone in the vicinity of the failed connector.
The amount of heat generated in an overhead conductor is proportional to the resistance of the conductor and to the square of current being carried on the conductor. The ultimate temperature of the conductor or a connector is dependent upon the heat generated in the conductor minus the heat lost through convection, radiation, and conduction.
The principal function of a connector is to secure two conductors together and to carry the electrical and mechanical load from one conductor to the next over its full service life. The electrical load of conductors may vary daily from no load to full load and, at times, the conductors may be overloaded. The temperature of the conductors and connectors will therefore vary based upon the heat generated in the conductor and by the ambient temperature and other weather conditions. Properly installed, healthy, overhead power line connectors typically have less resistance and more surface area for heat dissipation, therefore, conductors typically have a normal operating temperature that is less than the normal operating temperature of the conductor.
To increase reliability and anticipate connector failures, connectors are monitored to determine their current integrity in an attempt to predict connector failures. Though many connectors last longer than 60 years; some fail early in service. Early connector failures may result from improper installation, for example. The failure mechanism is usually characterized as a thermal runaway of the connector sleeve due to high resistance. It is well accepted in the power industry that electrical resistance of a connector is a good indicator of degradation and remaining service life. The resistance ratio, a measure of the connector resistance to the conductor resistance, is most commonly used to determine if a connector has failed or will soon fail. Temperature measurements are also used. In laboratory tests, ANSI C119.4 defines connector failure as temperature of the connector exceeding that of the conductor. Both indicators, resistance and temperature, are employed in field inspections. Inspection crews typically assess these connector conditions by real-time methods that measure only the status of the connector during the test period.
There are currently two commercially available methods for monitoring the integrity of an overhead power connector. Current monitoring techniques are based upon the theory that the connectors display a gradual increase in resistance over time on their way to failure. According to this theory, instantaneous assessment of the condition of the connector should be effective to determine if the connector may fail. In one method, the temperature of the connector is measured using infrared (IR) measurement techniques. As previously discussed, temperature is an indirect measurement of the electrical resistance of the connector. As connectors fail, the resistance increases across the connector and should be indicated by a higher than normal temperature. However, the temperature measurement of the connector is affected by other factors such as wind and load on the line. IR temperature measurement identifies “hot” connectors by thermal imaging but results are poor if line load is low or the wind is blowing. Therefore, an instantaneous temperature reading by IR of a connector is not a reliable measure of integrity of the overhead power connector as temperature is only an indirect measurement of resistance.
A second method is to directly measure the resistance of the connector by a live-line micro ohmmeter. The constant measure of the micro-ohm resistance of the connector can then be monitored to determine the integrity of the connector. While direct measurement of resistance is not confounded by wind or low line current, it still suffers from the problems such as improper measurement due to human error and due to the intermittency of high resistance episodes in the connector. Direct resistance measurement, provided by instruments such as SENSORLINKS OhmStik®, incorporates the resistance ratio method.
Other methods have been proposed to monitor the integrity of the overhead connectors including visual examination and X-ray inspection. Visual examination is qualitative and only very degraded connectors are found. In-service X-ray has proven to be prohibitively expensive, and normally used only to detect strand breaks due to fatigue. All of these methods have one thing in common: they provide a snapshot assessment of the connector's condition but do not give any indication of its thermal history. If the conventional theory of connector failure is not correct, these methods would not be reliable to determine the integrity of the connector.
There is a need for a reliable systems and methods for determining and monitoring the integrity of an in-service overhead line connector.